Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining" volume 4 page 433 (Interscience Publishers, New York 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually be selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point .degree. F. Point .degree. F. Gal. ______________________________________ Ethylbenzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethylbenzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para-xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para- and ortho-xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes. At present, several xylene isomerization processes are available and in commercial use.
The isomerization process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
It will be apparent that separation techniques for recovery of one or more xylene isomers will not have material effect on the ethylbenzene introduced with charge to the recovery isomerization "loop". That compound, normally present in eight carbon atom aromatic fractions, will accumulate in the loop unless excluded from the charge or converted by some reaction in the loop to products which are separable from xylenes by means tolerable in the loop. Ethylbenzene can be separated from the xylenes of boiling point near that of ethylbenzene by extremely expensive "superfractionation". This capital and operating expense cannot be tolerated in the loop where the high recycle rate would require an extremely large distillation unit for the purpose. It is a usual adjunct of low pressure, low temperature isomerization as a charge preparation facility in which ethylbenzene is separated from the virgin C.sub.8 aromatic fraction before introduction to the loop.
Other isomerization processes operate at higher pressure and temperature, usually under hydrogen pressure in the presence of catalysts which convert ethylbenzene to products readily separated by relatively simple distillation in the loop, which distillation is needed in any event to separate by-products of xylene isomerization from the recycle stream. For example, the Octafining catalyst of platinum on a silicaalumina composite exhibits the dual functions of hydrogenation/dehydrogenation and isomerization.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethylbenzene to benzene and diethylbenzene, hydrocracking of ethylbenzene to ethylene and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethylbenzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethylbenzene approach to equilibrium.
Concurrent loss of ethylbenzene to other molecular weight products relates to % approach to equilibrium. Products formed from ethylbenzene include C.sub.6.sup.+ naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylene isomerization reaction is much more selective than ethylbenzene conversion, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethylbenzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethylbenzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
A different approach to conversion of ethylbenzene is described in Morrison U.S. Pat. No. 3,856,872, dated Dec. 24, 1974. Over an active acid catalyst typified by zeolite ZSM-5 ethylbenzene disproportionates to benzene and diethyl benzene which are readily separated from xylenes by the distillation equipment needed in the loop to remove by-products. It is recognized that rate of disproportionation of ethylbenzene is related to the rate of conversion of xylenes to other compounds, e.g. by disproportionation.
In the known processes for accepting ethylbenzene to the loop, conversion of that compound is constrained by the need to hold conversion of xylenes to an acceptable level. Thus, although the Morrison technique provides significant advantages over Octafining in this respect, operating conditions are still selected to balance the advantages of ethylbenzene conversion against the disadvantages of xylene loss by disproportionation and the like.
A further improvement in xylene isomerization, as described in application Ser. No. 818,171, filed July 22, 1977, utilizes a combination of catalyst and operating conditions which decouples ethylbenzene conversion from xylene loss in a xylene isomerization reaction, thus permitting feed of C.sub.8 fractions which contain ethylbenzene without sacrifice of xylenes to conditions which will promote adequate conversion of ethylbenzene.
That improved process utilizes a low acid catalyst, typified by zeolite ZSM-5 of low alumina content (SiO.sub.2 /Al.sub.2 O.sub.3 of about 500 to 3000 or greater) and which may contain metals such as platinum or nickel. In using this less active catalyst the temperature is raised to 800.degree. F. or higher for xylene isomerization. At these temperatures, ethylbenzene reacts primarily via dealkylation to benzene and ethane rather than via disproportionation to benzene and diethylbenzene and hence is strongly decoupled from the catalyst acid function. Since ethylbenzene conversion is less dependent on the acid function, a lower acidity catalyst can be used to perform the relatively easy xylene isomerization, and the amount of xylenes disproportionated is eliminated. The reduction of xylene losses is important because about 75% of the xylene stream is recycled in the loop resulting in an ultimate xylene loss of 6-10 wt. % by previous processes.
Since most of the ethylbenzene goes to benzene instead of benzene plus diethylbenzenes, the product quality of the improved process is better than that of prior practices.
The improved process also allows greater flexibility with respect to charge stock. Since ethylbenzene conversion is relatively independent of isomerization, high ethylbenzene containing charge stocks can be processed, which means that charge stocks from thermal crackers (about 30 wt. % ethylbenzene) can be used as well as conventional stocks from reformers. In addition, dealkylation of C.sub.2.sup.+ alkyl groups is favored since the temperature is above 800.degree. F. As a result, paraffins in the charge stock will not alkylate the aromatic rings eliminating xylene loss via this mechanism. Thus, the improved process can process paraffins in the charge by cracking them to lighter paraffins eliminating the need for Udex Extraction. Finally, a small portion of the cracked fragments are recombined to form new aromatic rings which results in a net increase of aromatic rings.
The major raw material for p-xylene manufacture is catalytic reformate prepared by mixing vapor of a petroleum naphtha with hydrogen and contacting the mixture with a strong hydrogenation/dehydrogenation catalyst such as platinum on a moderately acidic support such as halogen treated alumina at temperatures favoring dehydrogenation of naphthenes to aromatics, e.g. upwards of 850.degree. F. A primary reaction is dehydrogenation of naphthenes (saturated ring compounds such as cyclohexane and alkyl substituted cyclohexanes) to the corresponding aromatic compounds. Further reactions include isomerization of substituted cyclopentanes to cyclohexanes, which are then dehydrogenated to aromatics, and dehydrocyclization of aliphatics to aromatics. Further concentration of aromatics is achieved, in very severe reforming, by hydrocracking of aliphatics to lower boiling compounds easily removed by distillation. The relative severity of reforming is conveniently measured by octane number of the reformed naphthas, a property roughly proportional to the extent of concentration of aromatics in the naphtha (by conversion of other compounds or cracking of other compounds to products lighter than naphtha).
The conventional techniques make BTX available from the "gasoline pool" of the petroleum fuels industry. This is an unfortunate result, particularly under present trends for improvement of the atmosphere by steps to reduce hydrocarbon and lead emissions from internal combustion engines used to power automotive equipment.
By far the greatest amount of unburned hydrocarbon emissions from cars occurs during cold starts while the engine is operating below design temperature. It has been contended that a more volatile motor fuel will reduce such emissions during the warm-up period. In addition, the statutory requirements for reduction and ultimate discontinuance of alkyl lead anti-knock agents require that octane number specifications be met by higher content of high octane number hydrocarbons in the motor fuel.
The net effect of the trends in motor fuel composition for environmental purposes is increased need for light aromatics to provide high volatility and octane number for motor gasoline. Present practices for supply of BTX to the chemical industry run counter to the needs of motor fuel supply by removing the needed light aromatics from availability for gasoline blending.
It is known that acid zeolites are very effective for disproportionation of alkyl aromatic compounds. See Frilette et al. U.S. Pat. No. 3,506,731, Wallace et al. U.S. Pat. No. 3,808,284 and Inoue et al. U.S. Pat. No. 3,671,602. The latter has shown that heavier aromatics, e.g. tri-methyl benzenes may be disproportionated to BTX and C.sub.10 + aromatics. The problem with that course is that a substantial portion of the product is C.sub.10 + aromatics which boil &gt;350.degree. F., which is at the upper limit or above the gasoline range and has little or no value as chemicals.
Typical processes for solving this problem by generation of BTX from heavy reformate, primarily by cracking off side chains of two or more carbon atoms, are described in U.S. Pat. Nos. 3,945,913, 3,948,758 and 3,957,621.
In a typical operation according to U.S. Pat. No. 3,945,913, heavy reformate is introduced to the xylene recovery/isomerization loop to blend with the stream of xylenes poor in P-xylene from the separation step. The conditions in the isomerization reactor are conducive to disproportionation. That feature makes it desirable to recycle the C.sub.9 + fractions to generate additional xylenes by conversion of, e.g. trimethyl benzene. See FIG. 2. The process of U.S. Pat. No. 3,957,621 also involves addition of a heavy reformate to the loop, the patent noting the greater stability at high temperature of zeolite ZSM-5 having a high silica to alumina ratio. Here again, the C.sub.9 aromatics are recycled to the reactor.